A modeling study found that global warming and demographic shifts toward Sun Belt cities will likely contribute equally to greatly increased exposure of people to extreme heat later this century. The projections are tallied in annual person-days of exposure to extreme heat, comparing the period 1971–2000 to the period 2041–2070. Person-days are calculated by multiplying the number of days when the temperature is expected to hit at least 95 degrees by the number of people who are projected to live in the areas where extreme heat is occurring. (Larger version)Credit UCAR.edu

Updated, 4:04 p.m. | A valuable study published this week in Nature Climate Change projects that exposure to extreme heat in the United States is likely to rise enormously by mid century, driven equally by demographic shifts boosting Sun Belt populations and projected changes in heat waves in a warming climate.

Seth Borenstein at the Associated Press has written a nice summary of the research, undertaken by a multi-disciplinary team at the National Center for Atmospheric Research in Boulder, Colo., and the City University of New York. Here’s the nut:

Between 1970 and 2000, the U.S. averaged about 2.3 billion person days of extreme heat each year. But between 2040 and 2070 that number will be between 10 and 14 billion person days a year, according to the study.

The biggest projected increases in person days is the Texas, Oklahoma, Louisiana, Arkansas census region where by mid-century heat exposure will increase by 2.7 billion person days. Right behind is south Atlantic region of Florida, Georgia, South Carolina, North Carolina, Virginia, West Virginia, Maryland, Delaware and Washington, D.C., where heat exposure is projected to increase by 2.2 billion person days. New England gets off the easiest with an increase of only 71 million person days. [Read the rest.]

In a news release from the National Center for Atmospheric Research, Brian O’Neill, an author of the study (and someone who has long focused on the interplay of population and climate change), stressed the importance of considering the interplay of societal patterns and climate patterns in gauging evolving risks:

Both population change and climate change matter…. If you want to know how heat waves will affect health in the future, you have to consider both.

The news release has much more background on the study and underlying issues, including this:

Extreme heat kills more people in the United States than any other weather-related event, and scientists generally expect the number of deadly heat waves to increase as the climate warms. The new study, published May 18 in the journal Nature Climate Change, finds that the overall exposure of Americans to these future heat waves would be vastly underestimated if the role of population changes were ignored….

Regardless of the relative role that population or climate plays, some increase in total exposure to extreme heat is expected in every region of the continental United States. Even so, the study authors caution that exposure is not necessarily the same thing as vulnerability.

“Our study does not say how vulnerable or not people might be in the future,” O’Neill said. “We show that heat exposure will go up, but we don’t know how many of the people exposed will or won’t have air conditioners or easy access to public health centers, for example.” [Read the rest.]

O’Neill’s point is vital to consider.

As with other environmental hazards, from earthquakes to storms, any societal impact will always be a function of the severity and frequency of the danger (in this case periods of high heat), the number of people exposed and their capacity to withstand the threat.

In fact, careful records of human time budgets show that not only New Yorkers and Indians but also Californians, reputed nature enthusiasts, average only about one-and-a-half hours per day outside. Fewer than 5 percent of the population of industrialized nations work outdoors. In developing countries, the number is plummeting and should be below 20 percent globally by 2050. As Lee Schipper shows, lifestyles revolve around the household. The achievement of ten thousand years of human history is that we have again become cave dwellers — with electronic gadgets.

Addendum, 4:00 p.m. | On Twitter, some climate curmudgeons tried to imply the study is bunk because the continuing Sun Belt migration illustrates that we prefer hotter temperatures. But, to my eye, that simply shows the importance of O’Neill’s point above — that exposure doesn’t equate with vulnerability, if policies and lifestyles take it into account.

I encourage you to read “Some Like it Hot,” a Cato Institute piece from early 2014 by Chip Knappenberger and Pat Michaels, to get Cato’s viewpoint (framed around limited government) on the same dynamics the Nature paper explores:

As both people’s mobility and their ability to select the climate they prefer have increased throughout this past century, the core of the U.S. population has moved southward—into warmer climates. The overall migration of people into the southern “Sunbelt” states has created a temperature change over time for the “average American” that far outstrips the most pessimistic measurements of global warming for the past century, and rivals the projections for the next! [Read the rest.]

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By 2050 or so, the human population is expected to pass nine billion. Those billions will be seeking food, water and other resources on a planet where humans are already shaping climate and the web of life. Dot Earth was created by Andrew Revkin in October 2007 -- in part with support from a John Simon Guggenheim Fellowship -- to explore ways to balance human needs and the planet's limits.